This invention addresses a commonly encountered and long-standing problem in the field of semiconductor manufacture. The problem concerns structures which comprise both silicon and silicon dioxide or silicon nitride, where it is desired to etch the silicon portion of the structure. The silicon dioxide and/or silicon nitride portion(s) of the structure may be present as a mask or as a permanent structural feature in its own right. The problem is that the etch process used to etch the silicon also attacks the silicon dioxide and silicon nitride. It is highly desirable to improve the etch selectivity of silicon to silicon dioxide and silicon nitride, since improved selectivity of this kind reduces the thickness of silicon dioxide or silicon nitride which is required in order to successfully withstand the silicon etch process. This in turn reduces cost and also provides for design flexibility. Selectivity is herein defined as the ratio of the amount of target material (for example, silicon) etched to the amount that the silicon dioxide and/or silicon nitride is etched. Various proposals have been made in order to improve the etch selectivity by introducing various gases at various points in the etch process. U.S. Pat. No. 6,290,864 proposes the use of an admixture of an etchant and a non-etchant such as helium. GB2473851 discloses the use of hydrogen and U.S. Pat. No. 7,041,224 discloses the use of hydrogen and/or oxygen, in both cases used as part of an admixture with an etchant. WO2011/006895 proposes the use of oxygen and/or an oxidising gas such as nitrous oxide, ozone or nitrogen dioxide at various stages in the etching process. These oxidising gases may require additional heat to be effective.